This invention relates to a pneumatic radial tire (hereinafter referred to as a "radial tire") having a two-ply structure consisting of inner and outer carcass layers, and more particularly to a radial tire which improves simultaneously both maneuvering stability and comfort to drive.
A radial tire has been proposed in the past which disposes a steel cord reinforcing layer in a region extending from each bead to each sidewall to improve maneuvering stability of the radial tire, and thus improves rigidity in this region.
For example, Japanese patent application Kokai publication No. 57-18503 proposes a radial tire having a structure wherein a bead filler having a dynamic modulus of elasticity of at least 300 kg/cm.sup.2 is disposed in a region extending from the upper surface of each bead core to a sidewall, a steel cord reinforcing layer having a height, from a bead base, within the range of 50% to 75% of a sectional height of the tire is disposed outside the bead filler, a carcass layer is turned up from inside to outside of the bead core in such a manner as to wrap therein these bead filler and steel cord reinforcing layer and the cord angle of this reinforcing layer is from 45.degree. to 75.degree. with respect to the carcass cord, or in other words from 15.degree. to 45.degree. with respect to a tire circumferential direction.
However, the radial tire described above is not free from the following problems. Since rigidity of the side portion of this radial tire increases in all the longitudinal, transverse and circumferential directions of the tire, comfort of drive drops with the increase of rigidity of the side portion in the longitudinal direction, though maneuvering stability can be improved. In the case of tires having a low aspect ratio such as recent radial tires, impact is absorbed at the upper end of the sidewall because the profile of the tire increases rigidity of the belt portion. Therefore, this structure lacks sufficient impact absorption capacity, comfort of drive gets worse and worse durability of the tire drops.
On the other hand, Japanese patent application Kokai publication No. 49-43304 discloses a radial tire having a structure wherein a steel cord reinforcing layer whose cord angle is from 5.degree. to 15.degree. with respect to the tire circumferential direction is disposed in the region of 25% to 45% of the sectional height of the tire, in order to satisfy both maneuvering stability and comfort of drive. This radial tire reduces rigidity of the side portion in the longitudinal direction by reducing markedly the cord angle of the steel cord reinforcing layer to the tire circumferential direction and improves comfort of drive. However, since the cord angle is too small in this radial tire, a great deal of force is necessary for causing deformation of the tire during the process in which a green tire is bonded to a tire molding drum, molded and then vulcanized. Moreover, stress balance cannot be kept during vulcanization between the portion where the steel cord reinforcing layer is disposed and the upper end portion where the layer is not disposed so that production defects occur and tires having uniformity cannot be produced with a high level of productivity. Further, since the difference of rigidity is great between the tip portion of the steel cord reinforcing layer of the product tire and the carcass layer in the tire circumferential direction, separation is highly likely to develop at the tip of the reinforcing layer even after a relatively short period of use.